1. Field of the Invention
The present invention relates to a direct sequence spread spectrum (DSSS) transceiver employed in a wireless personal area network (WPAN) communication system, and more particularly, to a DSSS transceiver capable of easily reducing an influence of a frequency error by using a short code word in an integral form in a transmitter and employing a differential detection method in a receiver.
2. Description of the Related Art
In general, IEEE 802.15.4, which is physical layer standards of a wireless personal area network (WPAN) known as Zigbee, is a standard for a wireless local communication having merits such as low speed communication, low power consumption, and low price. As application fields, there are network and control fields such as building control, home network including various home appliances and lighting fixtures, etc, cooling and heating control, mutual-assistance control, and sensor networks.
A direct sequence spread spectrum (DSSS) communication method applied to WPAN communication systems communicates by spreading one signal symbol as a certain sequence.
In this case, in the DSSS communication method, when an original signal is inputted to a pseudo-random noise sequence, it is possible to obtain a spread band spectrum signal with decreased power density per frequency.
On the other hand, when a receiving side uses the same pseudo-random noise sequence, the original signal may be restored.
Such communication method has advantages such as excellent modulation efficiency, quick synchronization of signal, and a small number of interferences in band due to lower power density and is used in code division multiple access (CDMA). Also, in Zigbee communication according to IEEE 802.15.4 standards, such DSSS communication method is used.
FIG. 1 is a configuration diagram illustrating a conventional DSSS transceiver.
The DSSS transceiver of FIG. 1 includes a DSSS transmitter 10 spreading and transmitting data according to the DSSS method and a DSSS receiver 20 restoring the data from a signal from the DSSS transmitter 10.
The DSSS transmitter 10 includes a code mapping unit 11 mapping data in a corresponding symbol by 4 bits and converts the symbol into a corresponding code and a radio frequency (RF) transmitting unit 12 transmitting the code from the code mapping unit 11 over an RF carrier wave.
The DSSS receiver 20 includes an RF receiving unit 21 converting an RF signal from the RF transmitting unit 12 into a base band signal and converting the base band signal into a digital signal, a phase error compensator 22 detecting and compensating a phase error of the base band signal from the RF receiving unit 21, and a symbol detection unit 23 detecting the symbol from the base band signal from the phase error compensator 22.
Operations of the conventional DSSS transceiver are as follows.
The DSSS transmitter 10 puts N number of bit data into one group and converts the group into one of 2N of symbols having a code word length of 2(N−1).
For example, source bit data is bound up into a group by 4 bits, the group is converted into one of 16 symbols, and the symbol is converted into one of 16 bi-orthogonal code words having 8 codes.
In following Table 1, there is shown conversion relationship between bit data and a bi-orthogonal code word.
TABLE 1Source bit dataSymbolBi-orthogonal code word0000000000000000110101010100102001100110011301100110010040000111101015010110100110600111100011170110100110008111111111001910111010101010110011001011111000100111001211110000110113101001011110141100001111111510010110
The DSSS transmitter serial-to-parallel converts the bi-orthogonal code word into a code word whose order is an odd number and a code word whose order is an even number to form a bipolar signal. The bipolar signal is appropriately pulse shaping filtered and multiplied by cos wot and sin wot that are carrier high frequency element, respectively, in the RF transmitting unit 11 to modulate and transmit the same.
After that, on the other hand, the DSSS receiver 20 multiplies a received signal by cos wot by sin wot and converts the received signal into a base band signal. In this case, since carrier frequencies of the DSSS transmitter 10 and the receiver 20 are not precisely identical to each other, there occurs a frequency error distorting the base band signal. In the conventional method, a complicated frequency error compensator is used to compensate the frequency error.
Also, a leftover phase error is compensated by a phase error compensator. In the beginning of packet data, preamble data for symbol synchronization and certain data SFD notifying the beginning of payload data, which are used by a synchronizer to acquire the symbol synchronization and frame synchronization.
After acquiring the symbol synchronization, correlation operation on an input signal and 2N of symbols that are reference code words is performed by a correlation unit and a symbol corresponding to a maximum correlation value is determined to be a received symbol and converted into bit data.
Generally, in communication systems supporting mobility at high speed, since a frequency error and a phase error are changed while receiving data, a complicated frequency error compensator and phase error compensator are used to compensate the frequency error and phase error in real time.
However, in the case of WPAN communication system requiring a low price, microminiaturization, and lower power consumption, a frequency error does not occur in real time while receiving packet data since mobility is very low. Accordingly, it is inappropriate to use the complicated frequency error compensator.
However, in the conventional method, when the frequency error compensator and phase error compensator are not used, there is present performance deterioration due to the frequency error and phase error.